With the Carve 144 a good compromise size would be 46 cm, possibly 42 for the smaller sail and 48-50 for the 9.0. That is about the sensible range. Bigger fin for lighter winds and bigger sail, smaller fin when well powered up with the 7.5 sail.

They Hypersonic is quite a different design. It makes do with a lot smaller fins, maybe in the 30-35 cm range.

For best allround performance move your straps back a little, to the mid positions. That usually gives a more natural stance.

Straps forward will not help your upwind performance, but it may help you get accustomed to sailing a modern board. When you are comfortable with the forward, inboard positions on your 144 you should move the straps back and out for better speed and upwind performance. I.e. the straps should be where they are in the picture here:

As PG notes, for the hypersonic you can use a somewhat shorter fin than recommended; perhaps 6 cm shorter, but it works ok with normal length fins, too. When I had a hypersonic it came with 2 fins; a 34 cm and a 44 cm. I used the 44 cm fin with sails 7.5 - 9.4, and I used the 34 cm fin with sails 5.2 - 6.6. I don't know if you've sailed the hypersonic much yet, but I have to warn you that it's a very difficult and technical ride which is sensitive to where you put the mast base, how you rig the sail, what size fin you have, where the footstraps are, etc. It will take a lot of trial and error to figure out what settings work best for you.

Hi windfrancois,
I think the current wisdom on "surface finishes" for windsurf gear/hi speed sailing dinghys/huge racing yachts is that a polished surface is too smooth and actually cuts down your speed.
Using a "figure 8" pattern with 400 wet/dry abrasive cloth is about the best you can do.
Some folks like a straight pattern in line with the water flow.
The idea is to create a little "tooth" (microscopic maybe, but "tooth" just the same) to hold a boundary layer of water molecules on the surface.
Testing has shown that water has less "friction" on a fully wetted out boundary layer than on a polished surface with no boundary layer or "wet out".
Surface finish can have a positive effect on preventing spin out, but mostly spin out
is due to imperfections in either the foil or the planform of the fin.
Hope this helps,

Thank you Roger, I was not sure that this concept was still on the edge. I remember some experiments about a membrane on sailboats with microscopic triangular channels to retain more water layer do you know something about this research.